Basic aspects of the clutch type of focus herein are generally known. To that end, and for easier understanding of the invention, a known form of such clutch type will be described below in detail with reference to FIGS. 1-9 hereof. Such clutch is denoted in its entirety by the reference number 30 and is usually included in a drive 20. The drive 20 comprises a driving shaft 22, connected to a motor, and a driven shaft 24, connected to the mechanical means which must be moved, typically the driving wheels.
As schematically shown in FIGS. 1-5, the drive 20 comprises in a manner known per se a control device 26 which is movable between a working position and a rest position and which allows the operator to connect or disconnect the driving shaft 22 and the driven shaft 24. This possibility of connecting and disconnecting the two shafts is provided precisely by the clutch 30.
Moreover, the drive 20 as a whole is preferably designed to define a ratio between the angular speed ωm of the driving shaft 22 and the angular speed ωc imparted to the driven shaft 24.
The clutch 30 (see FIGS. 8 and 9) comprises a driving bush 34 which is rigidly connected to the gear wheel 32 by means of axial pins 324. The driving bush therefore is rotated directly by the motor via the driving shaft 22 and, for example, a worm (not shown in the attached figures) which engages the gear wheel 32.
Next to the driving bush 34 there is a driven bush 36 which is rigidly connected to the driven shaft 24 by a pin 364. A coil spring 38, the ends 380 and 382 of which project radially outwards, is wound around the two bushes 34 and 36. A first collar 40 is arranged around the coil spring 38 and comprises a first recess 400 adapted to receive the first end 380 of the coil spring 38. The first collar 40 also comprises, on its radially outer surface, cam surfaces 404. A second collar 42 is also arranged around the first collar 40 and comprises a second recess 420 adapted to receive the second end 382 of the coil spring 38. The second collar 42 also comprises, on its radially inner surface, cam surfaces 424 which are adapted to cooperate with the cam surfaces 404 of the first collar 40.
In the rest configuration of the drive 20, i.e. when the operator does not operate the control device 26, the internal diameter of the coil spring 38 is slightly greater than the external diameter of the two bushes 34 and 36. In such configuration a fork (not shown in the drawings) pushed by a spring (not shown in the drawings) maintains the second collar 42 axially close to the first collar 40. The cam surfaces 424 and 404 convert the axial relative movement into a circumferential relative movement of the first collar 40 and the second collar 42. This rotation is also transmitted to the ends 380 and 382 of the coil spring 38. More specifically, the rotation thus obtained is in the direction of unwinding of the coil spring 38 and therefore results in an increase in the internal diameter of the latter. The coil spring 38 therefore surrounds the two bushes 34 and 36, maintaining them independent of each other. The rotational movement of the driving shaft 22 therefore causes rotation of the gear wheel 32 and, along with it, the driving bush 34 which rotates freely inside the coil spring 38. In this configuration, which is schematically shown in FIGS. 1 and 2, no other part of the clutch 30 is rotated.
Conversely, when the user decides to bring drive 20 in the working configuration, he acts on the control device 26 which releases the fork (not shown in the drawings) such that the second collar 42 axially moves away from the first collar 40. As a matter of fact, without the push of the fork, the coil spring 38 tends to go back to its non-deformed state and the relative rotation between its two ends 380 and 382 implies a circumferential relative movement between the first collar 40 and the second collar 42. The cam surfaces 424 and 404 convert the circumferential relative movement into an axial relative movement by which the two collars part from each other. More specifically, the rotation thus obtained is in the direction of winding of the coil spring 38 and therefore results in a reduction in the internal diameter of the latter. The coil spring 38 therefore tightens around the two bushes 34 and 36, rigidly connecting them together by means of friction. Since the driven bush 36 is rigidly connected to the driven shaft 24, the latter is also caused to rotate. It should be noted that the rotation of the driven bush 34 is usually oriented in such way that it also tends to wind the coil spring 38, thus increasing the friction effect. This configuration is schematically shown in FIGS. 3-5.
When the operator then decides to set the drive 20 back to the rest configuration, he/she again operates the control device 26 so as to re-establish the axial thrust on the second collar 42. This thrust therefore allows the collars 40 and 42 to return into their initial positions, also bringing the ends 380 and 382 of the coil spring 38 into their respective initial rest positions. The relative rotation resulting therefrom is in a direction opposite to the direction of winding of the coil spring 38 and therefore results in an increase in the internal diameter of the latter. The coil spring 38 therefore ceases to be tightened around the two bushes 34 and 36, so that they become again independent of each other.
The drive 20 which comprises the clutch 30 described above is contained inside a housing 28 which comprises a box 280 and a lid 282 and from which housing 28 only the driving shaft 22, the driven shaft 24 and the control device 26 protrude. This configuration allows very small overall dimensional values to be obtained, in particular with regard to the power transmitted. Moreover, with this particular type of clutch it is possible to achieve the result that the force required to operate the control device 26 is entirely independent of the power transmitted, the force of the coil spring 38 and/or any other force associated with operation of the clutch. In practical terms, the control device 26 is usually provided with its own return spring, the force of which is designed to provide the user with a clear indication of the return action, but without this however creating unnecessary fatigue.
The clutch 30 described above, while being widely used, is not without defects. Firstly, as seen above, the clutch 30 as a whole is axially constrained along the driven shaft 24 only by means of the pin 364 of the driven bush 36. This has the effect that the walls of the housing 28 play a more important role than simply that of containing the components and the associated lubrication oil. The walls of the housing 28, as is referenced in FIG. 5 and which can be further seen in FIG. 7, define a constraint in the axial direction for the components of the clutch 30 which bear against it by means of special washers 23. In addition, it must also be considered that the particular transmission of the movement by means of the worm and the gear wheel 32 gives rise to an axial thrust along the driven shaft 24. This thrust is such that the bearing action of the clutch against the walls of the housing 28 is not only fortuitous, but on the contrary systematic, constant and relatively energetic during operation of the drive 20.
As a result, with prolonged operation of the drive 20, a considerable amount of wear on the walls of the housing 28 and/or the washers may be noted. This wear usually gives rise to axial play which, in the long run, has an adverse effect on operation of the clutch 30 and of the entire drive 20.
It has been noted, in fact, that an elongation of the coil spring 38 in the axial direction results in a reduction in the torque which may be transmitted from the driving bush 34 to the driven bush 36 and therefore, ultimately, a reduction in the torque which may be transmitted by the drive 20 as a whole. However, the most serious malfunction which may occur as a result of the formation of this axial play is that a coil of the coil spring 38 may become wedged between the two bushes 34 and 36. In this case, operation of the drive 20 is definitively compromised and the drive must be replaced.
There are also a number of drawbacks of a logistical nature. As may be noted from FIGS. 5 to 7, the box 280 comprises two holes for receiving the driven shaft 24. The holes are formed in the wall of the box so as to be completely contained within it. The shape of these holes allows insertion of the shaft 24 only in the axial direction and a precise assembly sequence of the drive 20 is required. Firstly, the entire clutch 30, with the sole exception of the driven shaft 24, must be assembled. Then the clutch 30 must be positioned inside the box 280. At this point the shaft 24 may be inserted axially, in succession, through a first hole in the box 280, through the clutch 30 and through the second hole in the box 280. Only at this point may the pin 364, which keeps the whole assembly in position, be inserted. Before insertion of the pin 364, the clutch 30 is subject to undesirable disassembly operations which make the assembly step relatively complex. Then the driving shaft 22 with the respective worm and the control device 26 with the respective fork are mounted, and, once the lubrication oil for the mechanical components has been added, the lid 282 is mounted so as to close the box 280 of the housing 28.
From this assembly sequence it can be immediately understood why there are no other axial retaining means apart from the pin 364. Moreover, the drive 20 described above must necessarily be provided in its definitive configuration because this is the only one able to ensure sufficient resistance to accidental disassembly.
A further drawback of this known constructional form of the clutch 30 arises from the need to provide a hole for the pin 364 in the driven shaft 24. Following correct definition of the dimensions of the pin 364, it is required in some cases to form a hole with a diameter equal to nearly one third of the diameter of the driven shaft 24. The resistant cross-section of the driven shaft 24 is therefore locally reduced and the shaft as a whole is significantly weakened (see in this connection FIG. 9).
The object of the present invention is therefore to overcome at least partially the drawbacks mentioned above with reference to the prior art.